It is known that the compositions of coating materials, printing ink, adhesives, and the like may contain as one component thereof an unsaturated compound which cures on irradiation. Such a compound in the composition polymerizes to cure when the composition is irradiated with electromagnetic waves such as visible rays, ultraviolet rays, and X-rays, or particle rays such as electron rays, neutron rays, and alpha rays. The rate of polymerization is extremely increased when the irradiation is performed in the presence of a composition of polymerization initiator. These technologies are disclosed in U.S. Pat. No. 3,551,235, 3,551,246, 3,551,311 and 3,558,387, Belgian Patent No. 808,179, and Japanese Patent Application (OPI) No. 110781/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). These technologies provide products which have superior flexibility, chemical resistance, antiabrasion, gloss, adhesion and hue. On the other hand, they have some problems. That is, the sensitivity of the composition is so low that image formation requires a long exposure time. The longer the exposure time, the greater the chance that the apparatus will be moved. Even the slightest movement can deteriorate the quality of the image. If the energy of the light source or particle-ray source is increased to compensate for the low sensitivity, a secondary problem of heat occurs. The coating film of the composition might be deformed or degraded by this heat. Extensive research has been conducted to overcome the above-mentioned disadvantages by increasing the sensitivity of the photopolymerizable composition.
There is disclosed in Japanese Patent Application (OPI) No. 4126/72 a photopolymerizable composition containing an acridine or phenazine compound as a photopolymerization initiator. Although 9-phenylacridine, 9-p-tolylacridine, and 9-acetylaminoacridine are disclosed as having high sensitivity, this publication teaches that their sensitivity is not yet satisfactory. U.S. Pat. No. 3,479,185 teaches that 2-mercaptobenzimidazole, 2-mercaptobenzooxazole, and 2-mercaptosuccinic acid are useful as a free-radical generator in a photopolymerizable composition containing 2,4,5-triphenylimidazole dimer composed of two lophine residues. It was found, however, that the mercapto compound alone is not useful as a photopolymerization initiator of the photopolymerizable composition.